Method of treating clay



June 6, .1967 '5.' c. LYNs 3,324,208

METHOD oF-TREATING OLAY Filed June 5. 1966 H2O "cLAY FILTER CAKE (DF) (DFDF) DRIER :I

PUGMILL H2O I EXTRUDER DlsPERslNG CHEMICAL SLURRY (DFD) CONTROL TANK g COARSE PARTICLE cLAY sLuDGE (DFD) I I po I FINEs L{DRY, coARsE- PARTICLE 22\ cLAY, sea-5% H2O, FLoccED ID--II I I I I i FLoccuLATloN 28 24 I v 'f FIGI INVENTOR.

SAN FORD C. LYONS BY ATTORNEYS I I I I I I I I I I I I /BLUNGER I I I I I I I I I I United States Patent fitice 35324268 Patented June 6, 1967 3,324,208 METHOD OF TREATING CLAY Sanford C. Lyons, Bennington, Vt., assignor to Georgia Kaolin Company, Elizabeth, NJ., a corporation of New `Iersey Filed .Inne 3, 1966, Ser. No. 562,034 2 Claims. (Cl. 264-37) This nvention relates to an improvement in treating kaolinitic clay to improve its whiteness. The application is a continuation-in-part of my copending applications Ser. No. 764,395, filed Sept. 30, 1958, for Improved Kaolin Product and Process for Making Same, and Ser. No. 385,- 982, filed July 29, 1964, for Method of Treating Clay. The process described in the earlier application for producing clay of exceptonal whiteness comprises classifying washed kaolin to remove most of the fines, that is, particles having an equivalent spherical diameter of 2 microns or less, a tolerance of about 20% of fines being permissible, adjusting the consistency of the coarse fraction to a moisture content of from 17% to 23% depending on the physical characteristics of the clay which is being operated on, the critical consistency being that which requires a pressure of at least 350 pounds per square inch to force the clay through a die having holes 3/16" in diameter, and extruding under such pressure ("superstruding) the clay mass through a die having holes between 1/16" and 7/6" diameter. The shearing forces eXerted on the "stacks (coarse particles) in this process result in the delamination of many of such particles and a substantial increase in the percentage of fines in the clay mass. This extrusion step may be repeated, three such passes being good practice. The extruded mass is then diluted and further deflocculated if necessary, sufiiciently to be classified, the resultant fines fraction being found to have a high degree of whiteness, due to freshly exposed white faces in the plates resulting from the delamination of many of the stacks.

One of the principal economic and technical drawbacks to this process is that there is no known method for separating the freshly delaminated fines from the remaining undelaminated kaolin stacks while the material is at solids concentration higher than about 40%. Since the clay issuing from the extruder (including both fines and coarser residual remnants of kaolin stacks) has a solids concentration of about 80% or more, it must be diluted for reclassification and the resultant slurry must also be deflocculated by the addition of a suitable dispersing chemical to 'facilitate this Classification step. The coarse fraction resulting from this classfication (i.e., particles greater than 2 microns) may be again subjected to high-pressure' extrusion.

If centrifugation is employed for classifying, the resultant coarse fraction issues from the centrifuge with a customary 68% of solids, that is, with a consistency like that of soft putty. This is much too soft for further delamnation by extrusion. The moisture content must be reduced to the specified moisture content of 17% to 23% either i by the removal of some of the water or by the addition of dry clay. The clay must also be transferred from the centrifuge back to the extruder mechanism. Owing to the size, weight and Operating conditions of the various apparatus units through which the clay must pass, (e.g., the occasional transitory unbalancing of a Bird centrifuge when passing through critical speeds in accelerating or decelerating which makes it necessary to mount the centrifuges on massive foundations) it is practically infeasible to arrange the apparatus in such a way that this coarse fraction of clay issuing from the centrifuge can be returned to the extruder by any method other than pumping. The practice has been to dilute the mass to about 35% solids, so that the slurry can be pumped, then to flocculate it by the addition of alum or acid so that it can be filtered, then to dewater the slurry by means of a rotary vacuum filter which yields a cake having about 68% to 69% solids, this cake being then fed to a conventional drier to reduce the moisture content to about 5%. This dried clay is delivered to a feeding device, such as a pugmill, where water is carefully added to bring it to the proper consistency for superstrusion, that is, for forcing the same under high pressure through small openings in a die to delaminate many of the large particles or "stacks. This practice requires large, complex and expensive apparatus for treating commerical quantities of clay, and also requires operators, fuel and sundry Supplies. In addition, dispersing and flocculating chemicals must be added to the chemicals already in the clay mass when it issues from the centrifuge. It may be observed that the conveying of clay at either of the two ends of the moisture-content Spectrum, that is, dry clay or fiuent slurry, is no problem, but the conveying of clay in the twilight zone as it is, for example, when issuing from a Bird centrifuge, is very difiicult owing to the stickiness of such clay. According to the present invention, the apparatus which has heretofore been required for transferring the coarse fraction from the centrifuge back to the extruder can to a substantial eXtent be by-passed with resultant economy in operation and a better endproduct.

Chemical salt residues are objectionable in many kaolin products, but particularly in those destined Ifor use in the paper-coating industry, because these salts promote an agglomerative tendency of the clay particles which is difiicult to overcome in the preparation of the paper-coating composition. In processing kaolin clay from the crude state to the finished product the addition of chemicals for fiocculating or dispersing the clay is necessary, but each addition of a chemical to reverse the effect of the previously added chemical increases the size of the dose required for the next treatment. Hence, it is important to keep to a minimum the number of applications of chemicals to the clay. Any reduction in the number of chemical treatments of the clay results not only in an improved product but also in a substantial saving in the cost of the chemicals employed.

The coarse particle fraction of washed kaolin which has been subjected to a delaminating operation and has been classified by centrifugation, as hereinbefore described, issues -from the centrifuge with a solids content usually of 68%. At this consistency, the coarse particle fraction is conspicuously dilatant, that is, characterized by a sharp rise in viscosity when agitated. This material is sluggishly fiuid when undisturbed but when pressured becomes almost solid. Hence any pump of which I have knowledge and which is capable of pumping liquids effectively Will be completely blocked if this dilatant material is introduced into it. This characteristic is apparently due to the fact that the material consists predominatly of relatively large (greater than 2 microns, equvalent spherical diameter) plates. When the water in which they are suspended is insufficient to permit the plates to turn freely, they catch on one another when pressure is applied. A highly magnified analogy would be a mass of poker chips suspended in a limited amount of liquid having a specific gravity only slightly less than that of the chips. Very few substances are known to have substantial dilatancy. For example, if the moisture content of the coarse fraction of delaminated clay be increased by even a few percent, the clay loses most, if not all, of its dilatancy. Experiments performed with a Brookfield viscosimeter on several liquids produced the following results.

1 Off Scale.

The Dilatancy Indices above are derived by dividing the figures for dilatancy at 100 r.p.m. by the figures for dilatancy at 10 r.p.m. Thus a liquid of so-called Newtonian character will have a Dilatancy Index of 1, its viscosity being the same for all speeds of the viscosimeter. A liquid having a Dilatancy Index less than unity is thixotropic. Such liquids become more fluent when agitated by stirring. A liquid having a Dilatancy Index greater than unity is dilatant. The only specimen listed in the foregoing table which is strongly dilatant is the coarse particle clay slurry having 68% solids.

While dilatant material such as a coarse particle clay slurry having 68% solids cannot be pumped by any pump known to me which is capable of effectively pumping water, I have discovered that such slurry can be successfully pumped by the use of a rotary displacement-type pump in which an unusual clearance of about 0.010 is provided between the rotors and between them and the casing in which they are housed.

These clearances which are excessive for ordinary use, permit the relatively free movement of the pump rotors within their casing even when pumping dilatant clay slip, yet the relatively low fluidity of the dilatant clay prevents excessive "slippage or recycling of it through the comparatively large (i.e., 0010") clearance gap which Would permit so much slippage of any ordinary liquid or thixotropic substances as to render the pump virtually useless for such service. For example, if a heavy grease which is non-fiuent at ordinary temperatures should be pushed into such a pump the shearing effect of the pump elements on the grease in the pump would liquefy it to such an extent that serious slippage would occur through the clearances. It has been found that while these pumps, with a wide clearance such as this, will hardly pump water at all (due to this slippage), the intervening film of moist clay in the clearance gap seems to lubricate the system and sets up just enough transistory dilatant viscosity to obviate any serious "slippage or recycling. In this way, the coarse fraction from the centrifuge can be forwarded directly to the pugmill or other device for feeding the extruder by means of a simple pipeline, which obviates almost completely the necessity for belt conveyors, bucketelevators or other conveying instrumentalities, with their concomitant sizable structural and engineering problems, installation costs and maintenance expenses.

Dried clay can be introduced into the pugmill with this coarse fraction in carefully Controlled quantities to give the mass the correct moisture-to-solids proportion for effective 'superstrusion- The dried clay will have to be obtained by some such method of dryin'g as hereinbefore described, but for the production of a given tonnage of end product the amount of clay which has to be dried to 5% moisture is far less than the total quantity treated. This reduction in the proportion of dried clay which must be blended with the centrifugally concentrated coarse-particle clay can be as high as nearly 50% of the total feed to the extruder. In a typical commercial installation, this saving can amount to more than four tons per hour (dry basis) of slurry-clay. The conventional equipment which would be necessary, eg., vacuum filters, driers, elevators, conveyors, etc., to provide this amount of normally dewatered clay would approximate $100,000 -not to mention the concomitant costs for drier and filter operators, fuel, and power.

My novel method of thus providing a substantial proportion of the feed clay for the superstrusion constitutes a major contribution to the economic efliciency and success of the superstrusion process first described in my U.S. Patent No. 2,904,267.

Yet another advantage of my novel 'method is an improvement in the quality of the product, i.e., in its lower content of chemical salt residues, which are known to exert an adverse effect upon certain of the use-properties of these new clays, e.g., disintegration, viscosity, emulsion stability, etc.

As hereinbefore mentioned, the normal processing of the coarse-particle centrifuge concentrates would call for their dilution with water, followed by treatment with a flocculating chemical which would neutralize the dispersing chemical already present herein. This interaction of chemical agents produces salts whose residues necessitate reintroduction of more dispersing chemical after the next superstrusion step.

Since the centrifuge concentrate as used in the instant process does not require fiocculation ahead of the extruder, and since it already contains its own component of dispersing agent from its prior fractionation, it virtually obviates the necessity for adding chemicals at this point.

On the drawing, FIGURE 1 is a flow diagram showing the progress of the clay from the extruder to a centrifuge and back to the extruder by the new process and (indicated by broken lines) by the prior process; and FIGUR'E 2 is a sectional view of a pump of a type that can be used in the practice of my new process.

In carrying out the process for producing clay of extra whiteness as described in said application Serial No. 764,- 395, raw clay is washed and then fractionated to reduce the percentage of the fines to about 20% or less which, in secondary clays, are always more or less discolored by surface impurities which cannot be entirely removed chemically. The residual coarse fraction is then dewatered by filtering, and the filter cake is sent to a drier 10 in which the moisture content is reduced to about 3%. In the process of classifying the clay it has been treated with a dispersing chemical before entering the centrifuge and with a flocculating chemical before getting to the filter. On the drawing these and later treatments are designated by the initials D and F indicating the kind and number of treatments the clay has had at various stages of the whole operation, the last letter indicating the condition of the clay at that point, i.e., dispersed or fiocculated.

The dry clay (flocced) from the drier 10 is fed to a pugmill 12 where carefully measured water is mixed with it so that when discharged into an extruder 14 it has the critical percentage of moisture, i.e., 17% to 23% or in the case of some foreign clays, as low as 15%. If, as indicated, moist coarse-particle clay is added to the dry clay, less free water will have to be added to bring the moisture of the clay in the pugmill to the critical percentage.

The clay which is "superstruded in the extruder as described in Patent No. 2,904,267, is discharged into a blun-ger where it is thoroughly mixed with water and a dispersing chemical to form a slurry thin enough for effective fractionation in a centrifuge or otherwise. The slurry is discharged into a control tank 18 to regulate the feed-rate of the clay slip into a centrifuge 20 or settling tank (not shown) to recover as much of the fines fraction as can economically be separated from the residue, the fines fraction being conducted away for further treatment.

According to the prior practice, the residual coarse fraction is recirculated to the extruder 14 by first reducing it to a slurry which can be pumped. The coarse fraction as it comes from the centrifuge has a solids content of 68%. Water is added to this in a miXin-g tank 22 to form a slurry with a moisture content of 60% or more. This slurry is pumped to another tank 24 near a filter 26, the slurry being flocculated in the tank 24 so that it can be filtered. From the filter it goes to a drier 28 which reduces its moisture content to about 5%. This dried clay in which the flocculating chemical predominates is fed to the pulpmill 12 where it is added to the dried clay from the drier and enough water is mixed in to adjust the moisture content to the percentage suitable for "superstrusion."

According to the present invention, instead of the rniX- ing tanks 22 and 24, the filter 26 and the drier 28, a positive displacement pump 30 is mounted near the centrifuge to receive therefrom the coarse-particle fraction. An example of the type of pump employed is indicated in FIGURE 2, this pump being widely known as a Viking pump with a -driven inner gear 3=2 and an outer idler 34 which are mutually eccentric, a fixed crescent-shaped seal being provided between the portions of the gears which do not mesh. For pumping the thick clay sludge .that comes from the centrifuge with 'a moisture content of 32%, clearances of about 0.010 are provided between the surfaces of the gear teeth and casing which ordinarily touch one another. This provides thin films of clay between the surfaces which usually slide upon each other in mutual contact when the pump is in motion, and the dilatant tendency of the slurry effectively seals the clearances which make the pump useless for pumping thin liquids such as water.

By eliminating the filter 26, the recirculated clay arrives at the pugmill 12 with a lower chemical content and with the dispersing |chemical predominating. This reduces the amount of dispersing chemical which will have to be added to the clay discharged from the exterior into the blunger 16.

I claim:

1. The method of moving through a pipe a dilatant coarse particle clay mass having a moisture content of about 32%, which comprises connecting said pipe to the discharge o rifice of a positive displacement rotary pump having a clearance of about 0.010" between the moving parts and stationary parts thereof, feeding such clay to the pump, and Operating the pump.

2. The process of recirculating to an extruder the coarse particle fraction of kaolinitic clay which has been extruded under conditions required for delamination of many of the coarse particles Originally in the clay and which has been diluted, deflocculated and centrifuged, pumpin'g the di latant coarse particle fraction as it comes from the centrifnge in deflocculated condition with about 32% moisture content to a mixture near the extruder by means of a positive displacement rotary pump having a clearance of about 0.01" between the moving parts and the stationary parts thereof, mixing said coarse particle fraction with suflicient dried clay to reduce the moisture content of the mass to the proper percentage for delaminating extrusion, and feeding the mixture to the extruder.

References Cited UNITED STATES PATENTS 9/1959 Lyons.

OTHER REFERENCES ROBERT F. WHITE, Primary Examner.

I. A. FINLAYSON, Assistant Exam'iner. 

1. THE METHOD OF MOVING THROUGH A PIPE A DILATANT COARSE PARTICLE CLAY MASS HAVING A MOISTURE CONTENT OF ABOUT 32%, WHICH COMPRISES CONNECTING SAID PIPE TO THE DISCHARGE ORIFICE OF A POSITIVE DISPLACEMENT ROTARY PUMP HAVING A CLEARANCE OF ABOUT 0.010" BETWEEN THE MOVING PARTS AND STATIONARY PARTS THEREOF, FEEDING SUCH CLAY TO THE PUMP, AND OPERATING THE PUMP, 